1. Field of the Invention
The present invention relates to a drive circuit that is connected between an electric power source and an electric load and a method of applying a high voltage test on the drive circuit.
2. Description of the Related Art
JP-A-2001-160748 or U.S. Pat. No. 6,392,463 B1, which is a counterpart of the former, discloses a drive circuit that includes series connected MOSFET type transistors (hereinafter referred to as MOSFET or MOSFETS) for high-side or low-side driving. The drive circuit includes a clamp circuit that protects the MOSFETS when a high voltage is applied to the drive circuit. Such a clamp circuit is a series circuit of a backflow preventing diode and a zener diode that is connected between the drain and the gate of an N-channel MOSFET for low-side driving.
In order to provide a drive circuit with a fail safe function, the same MOSFETS may be connected in series for high-side driving or low-side driving. In this case, the drive circuit can properly drive an electric load connected to the drive circuit even if one of the transistors short-circuits. However, if the clamp circuit that is disclosed in the above publication is merely connected to this drive circuit, both the MOSFETS may simultaneously turn on when a high voltage is applied to the drain terminal of the MOSFETS even if the gate signal is not applied to the gate terminal. In other words, it is impossible to keep the MOSFETS turning off when a high voltage is applied to the drain terminal.